The present invention relates to a process for the production of a flexible superconductor comprising a carbon fiber with a thin layer of a niobium compound of the general formula NbC.sub.x N.sub.y wherein the sum total of x+y has a maximum value of 0.9 to 1.0, y has a value of 0.60 to 0.95, and x has a value of 0.05 to 0.40, and with an external, highly conductive metal layer, wherein the carbon fiber is treated with niobium chloride, carbon compounds, and nitrogen or nitrogen compounds in the gaseous phase.
Niobium carbonitride, NbC.sub.x N.sub.y, is a well known high-temperature superconductor. With proper selection of values for x and y, the transition temperature, i.e. the temperature below which the carbonitride becomes superconductive, may be in the range of 18.degree. K.
Like other high-temperature superconductors, niobium carbonitride is brittle and thus can find only limited application in its solid form. This difficulty is avoided by producing a thin-layer of superconductor on a thin, flexible substrate.
There is, especially in the construction of large-scale superconducting magnets for hydrogen fusion, a requirement due to the occurring tensile stresses on the conductor that the conductor to be protected from a degradation of its superconducting properties.
To avoid these difficulties, it is known to deposit niobium carbonitride on carbon fibers by use of chemical vapor-phase deposition processes. To produce a niobium carbonitride coating on a substrate by chemical vapor-deposition (CVD), one would introduce, simultaneously, into a high-temperature furnace, a substrate, and a source, each, of niobium, nitrogen and carbon. Niobium pentachloride, NbCl.sub.5, is usually the niobium source, while nitrogen may be supplied by N.sub.2, NH.sub.3, (CH.sub.3).sub.3 N, or (CH.sub.3).sub.2 NH, and carbon may be supplied by CH.sub.4, or by one of the above mentioned amines (CH.sub.3).sub.3 N or (CH.sub.3).sub.2 NH which is also used as the nitrogen source. In another process when the substrate is a carbon filament, it is also possible to form the niobium carbonitride by reacting the carbon filament by simultaneous exposure of the carbon filament to NbCl.sub.5, H.sub.2 and N.sub.2. The temperature at which these processes takes place is often about 1200.degree. C., and can be in the range of 1400.degree. C.
A particular property of a superconductor which determines its usefulness, is its critical current density, that is, the amount of current that it can carry without reverting to its normal-conducting state. Due to the high manufacturing temperature of the NbC.sub.x N.sub.y -coated carbon fibers produced by chemical vapor deposition, the coated fibers exhibit a structure which is unsuitable for high current carrying values. The problem of controlling the critical current density is discussed in G. E. Pike et al, Applied Polymer Symposium, No. 29, pp. 71-81 (1976), and in W. D. Smith et al, Applied Polymer Symposium, No. 29, pp. 83-92 (1976).
Absolute requirements for the achievement of high current-carrying values at high magnetic fields are sufficiently small grain sizes of the cubic niobium carbonitride, and/or a fine distribution of other phases such as hexagonal and tetragonal niobium carbonitride or voids, within the cubic carbonitride on the fiber. However, the single-stage manufacture by a simultaneous deposition of all components at high temperatures (around 1200.degree. C.) promotes the formation of large grains with a low proportion of other phases.
Although the particle growth can be restricted by the use of additives such as SiCl.sub.4 during the manufacturing process, as discussed in W. D. Smith et al, Op. cit., this is an additional burden on the manufacturing process, and negative effects can be expected on the superconducting transition temperature.